near field probe

It seems like the Flex radios could be used as a spectrum analyzer with a near field probe to track down sources of RFI in the shack. Can anyone recommend a dyi design for a probe that would work in the HF bands? I've come across designs in the GHz range but nothing for HF yet. Thanks -- Keith KH6FQR

I set this thing up just to see how it worked and I got division by zero errors and a message saying that an outgoing call cannot be made since the application is dispatching an input-synchronous call.

Hi James, Thanks, I'll try that. I was just checking to see if anyone had any experience doing this already. It seems like the Flex could be a useful tool for this sort of thing. It is certainly capable of "seeing" much more RFI than my other radios ;-)

That's what Sears catalogs were for.On a technical side, why would the pickup coil need to be 50 Ohms when we are only trying to couple a few microvolts into the rcvr? we're not looking for maximum energy transfer by matching impedance. 73, Jim

The 6000 Series is an instrument capable of accurately displaying signals down to a range of -140dBm (20 nV) across a 50 ohm load. So if your trying to maximize the laboratory capability of the device you should use a 50 ohm load.

On the other hand, if you are just trying to roughly see what RFI spurs are around then you don't need a resonant circuit.

It took me less than 5 minutes to build a resonant probe so no big effort required

Just to be an absolute technonerd about this, I just spent the last hour playing with my vector network analyzer to tune the probe to exactly 50.0 ohms @14.200MHz.

I guess I have too much time on my hands this morning (more likely Jet Lag effects)...

Again... Thank you Keith for the idea and James for thinking of a simple probe solution.. I have already used the probe to find an RFI source that was bugging me and my other test instruments could not see...

Now I plan to play around designing a few different size probes...to see if I can get them into hard to reach places....

Now I know why I like this forum so much. Great ideas and shared information. Really stimulates my interest. What a super bunch of guys!!! Please share your designs with us.... I can see many uses for this. Jim

If you require a broadband response from your probe you do NOT want it to be resonant. It's common to use either an E field or an H field probe since different sources of RFI tend to favour one or the other (ie. capacitive or inductive coupling). A simple loop of wire forms an effective H field probe. Whilst simply stripping back the braid on some coax leaves the inner as an E field probe (in effect a little monopole antenna). Different geometries will give different levels of sensitivity and spatial response. Endless hours of fun

I have a probe that I made just as Paul suggests, I left the centre conductor wire insulated so that when I was probing around in the radios I would not short things out!! Worked great for reading on the frequency counter. Also made a few probes on a piece of 3/4" PVC pipe with an SO239 connector for change ability.